Known high pressure fuel pumps include a high pressure circuit which delivers highly pressurised fuel to a rail, and a low pressure circuit.
A nominal pressure within the low pressure circuit must be kept almost constant at all running conditions, regardless of running speed, room temperature, fuel demand etc. If pressure within the low pressure circuit exceeds a given threshold, this can lead to poor performance of the fuel pump, or even system failure.
To achieve a constant pressure in the low pressure circuit, it is known to use a low pressure regulator (LPR) as part of the circuit. The LPR regulates pressure within the low pressure circuit by dumping fuel into a return line when pressure exceeds a given threshold.
A known LPR 2 is illustrated in FIG. 1, and includes a body 4, a piston 6, and a spring 8, located in a spring chamber 10 formed in an interior of the body 4. The spring chamber 10, and a damping orifice 12, one end of which communicates with the spring chamber 10 and the other end with a return line 14, provide for damping of the piston 6 during movement.
Discharge port holes 16 provided in the body 4 are uncovered by the piston when its lift exceeds a certain distance. The LPR 2 operates in two modes; the mode in which it is operating at a particular time is determined by the location of the piston 6 with respect to the body 4, i.e. the lift of the piston 6. The two modes are hydraulic accumulator mode, and regulator mode.
The damping orifice 12 is located such that it is not obstructed by the piston 6 in any operational positon of the piston 6, such that a constant level of damping is provided by the damping orifice 12 and spring chamber 10.
The known LPR 2 acts like a mass-spring-damper system excited by an oscillating force. However, for large, and very fast pressure changes, oscillation of the piston 6 could be so rapid that the spring chamber 10 will only be partially filled, and therefore will be unable to provide damping of the movement of the piston 6.
The known LPR 2 could therefore run in a damped or an un-damped mode, depending on the sufficiency of filling of the spring chamber 10. Under certain conditions, when the LPR 2 transits from the damped to the un-damped mode, voids in the spring chamber 10 collapse, generating undesirable pressure spikes which propagate within the low pressure circuit.
An increased size of damping orifice 12 mitigates the problem of partial filling of the spring chamber 10 and resulting pressure spikes, however a larger damping orifice 12 reduces the damping effect of the spring chamber 10 when it is fully filled.